Metal-oxide-semiconductor field effect transistor (MOSFET) technology is currently the dominant semiconductor technology used for manufacturing ultra-large scale integrated (ULSI) circuits. As the gate length of the MOSFET is scaled down into the sub-30 nm regime for improved performance and density, the source and drain increasingly interact with the channel to sometimes gain influence on the channel potential. Hence, a transistor with a short gate length often suffers from problems related to the inability of the gate to substantially control the on/off states of the channel, which is often called short-channel effects.
Increased body doping concentration, reduced gate oxide thickness, and junction depths are some ways to suppress short-channel effects. However, for device scaling well into the sub-30 nm regime, the requirements for body-doping concentration, gate oxide thickness, and source/drain doping profiles become increasingly difficult to meet using conventional device structures based on bulk silicon substrates. Thus, alternative device structures that offer better control of short-channel effects are being considered to enable the continued scaling down of transistor sizes.
A highly scalable device structure that offers superior control of short-channel effects is a wrap-around gate structure for a transistor (a.k.a., surround-gate or gate-all-around transistor structure). A wrap-around gate structure typically has a gate that surrounds or wraps around a channel region. This structure effectively improves the capacitance coupling between the gate and the channel, as compared to conventional bulk silicon substrate transistor structures, double-gate transistor structures, and triple-gate transistor structures. With the wrap-around gate structure, the gate gains significant influence on the channel potential, and therefore improves suppression of short-channel effects. A wrap-around gate structure typically allows the gate length to be scaled down by about 50% more compared to a double-gate structure.
There are several different ways to implement a wrap-around gate transistor structure. For example, the transistor channel may be oriented vertically or horizontally. Many of the existing designs for horizontally oriented channels have a square or rectangular shaped cross-section. When the channel cross-section is rectangular or square, enhanced field effect at the corners of the rectangle may cause that part of the transistor to turn on earlier (i.e., having a lower threshold voltage) than parts of the transistor at the flat sides of the rectangular channel cross-section. This may result in a parasitic off-state leakage. Hence, a cylindrical channel cross-section is preferred over a rectangular channel cross-section.
Current attempts at obtaining a more circular channel cross-section are made by oxidizing the silicon beam forming the channel to round the corners of the rectangular channel cross-section. However, this method requires a large amount of oxidation, and hence a large amount of oxide formation, to convert the rectangular channel cross-section shape to a rounded or circular channel cross-section. Hence, there is a need for a way to manufacture a transistor channel having a rounded or circular cross-section shape without having to form excessive oxide about the channel.